Spectrally efficient digital modulation over radio channels requires the use of multilevel/multiphase signals. This type of signal is very sensitive to time-varying amplitude and phase distortion associated with land mobile communications, such as Rayleigh fading.
To improve the sensitivity of the transmission system without increasing its bandwidth, trellis coded modulation techniques are used. Historically, such a technique has been used to improve the transmission performance over channels where the main source of errors is additive noise. At the receiver, a decoder that uses the Viterbi algorithm with a Euclidean metric, such as .phi..sup.2, is used to recover the transmitted information from the trellis coded sequence. This type of decoder is optimum if the noise associated with the samples at the channel output is Gaussian and its samples taken at the symbol rate are uncorrelated. Neither of these conditions, however, applies to land mobile radio receivers. As a result, decoding by use of Viterbi algorithm with a Euclidean metric in a Rayleigh fading environment makes performance of the trellis coding scheme worse than its uncoded counterpart, due to rapid phase variation when the signal undergoes a deep fade.
Accordingly, there is a need for a method of decoding that limits the effects of rapid phase variation in deep fades.